Method for sputtering garnet compound layer

ABSTRACT

A method is disclosed for sputtering epitaxially a layer of stoichiometric garnet composition from a single target wherein the target is composed of a mixture of the separate components of the sputtered layer.

United States Patent [191 Cuomo et al.

. 5] June 3, 1975 Sadagopan, Scarborough, all of N.Y.

[73] Assignee: International Business Machines Corporation, Armonk, NY.

221 Filed: Dec. 29, 1972 21 Appl. No.: 319,589

[52] U.S. Cl 204/192; 204/298 [51] Int. Cl. C23c 15/00 [58] Field ofSearch 204/192 [56] References Cited UNITED STATES PATENTS 3,437,5774/1969 Kay et a1 204/192 3,573,099 3/1971 Moore et a1. 204/192 3,607,6989/1971 Kay et a1 204/192 OTHER PUBLICATIONS Primary Examiner-Oscar R.Vertiz Assistant Examiner-Wayne A. Langel Attorney, Agent, orFirmBernard N. Wiener [57-] ABSTRACT A method is disclosed forsputtering epitaxially a layer of stoichiometric garnet composition froma single target wherein the target is composed of a mixture of theseparate components of the sputtered layer.

lllustratively, both at a substrate temperature of approximately 450Cand at another substrate temperature between 800-850C, there wasobtained formation of a film of gallium substituted yttrium iron garnet(GazYlG). A target was made up of the desired stoichiometry with amixture of the individual oxides pressed to 85% of the compoundstheoretical density. Generally, the steps of the method are: (l)applying a radiofrequency bias to the substrate during sputtering toprevent the deposition of an easily resputtered component of the target;and (2) changing the power density to the target during deposition.

Specifically, a target was made up of a mixture of individual oxides Y OGa O Fe O which was pressed to 8.5% of its theoretical density.Exemplary films of stoichiometric composition were obtained with aradio-frequency bias on the substrate in the range approximately fromground to 100 volts and with power density to the target in the range ofapproximately 5 to 65 wattslin The stoichiometric ratio for thecomposition was Y Fe Ga,O, where 0 X 3, whereas the ideal stoichiometricratio of [Fe ,Ga /Y is 1.667. This procedure obtained stoichiometricratios approximately in the range 1.4 to 1.6.

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1 METHOD FOR SPUT'IZERING- GARNET COMPOUND LAYER BACKGROUND or THE.lNvliNTlON chiometric and the prior art accumulation rate wasrelatively poor. Target enrichment in the deficientmaterial has beendescribed at: i

a. Applied Physics Letters, 15, 256 ('196 9 Tak'ei, e t al. Thedeficient material is added to the desiredcornpounded material and mixedwith it. They used mixed oxide targets rich in bismuth oxide for'sputtering single crystal bismuth titanate film/The yield differe r icebetween the compound and the added deficient component is notpredictablefFurther, the added presence of the excess material shiftsthe target composition so that many phases are present. I g ,7

b. IBM J. of Resand Dev. 13., 696 (T969), by Sawatzky, et al., disclosescation deficiencies in garnet films, principally iron whereinthe irondeficiencies are supplemented in the filmswithithe use of an auxiliaryindependenttarget of iron.,,ln greater detail .films of gadolinium irongarnet Gd Fe O were prepared by radio-frequency sputtering butwere foundto have cation deficiencies, e.g., film depositedfroma stoichiometricgarnet target atsubstrate temperatures of 500C had about a.25% iron.deficiency,,while film deposited at 40C had about a 7% iron deficiency. Theloss in iron was compensated by the auxiliary iron target so thatstoichiometric composition garnet films were prepared. However, acomposition gradient was naturally introduced alongthe film surface.Thelatter method has problems of n'onuniformity. Since the addition ofthe deficient iron is across the film surface, vi.tis somewhat difficultbythis latter technique=to produce stoichiometric film with large area Yc. U.S. Pat. No. 3,607,698 by Kay, et al., discloses and claims themethod of making singlephase rare earth garnet thin films in asputtering apparatus comprising the steps of:. maintaining a garnetsubstrate-hav ing a (1 l l orientation directedtoward'the sputteringflux at a temperature lessthan 500C; maintainingan atmosphere in saidapparatus of at least IO percent pressure oxygen; sputtering materialfrom a source containing said rare earth iron garnet in bulk form tosaid substrate; crystallizing the depositedfilm which si-- Sci. andTech. 8, 512 1971') by J. L. vdssen:

thev film composition by radio frequency bias sputt'erin'gh'as beendiscussed at J. Vac.

" Although both mixed oxide targets and biasing of a growingfilmduringsputtering have been discussed in the priorart literature,illustratively, the use of a target of an enriched mixture of oxideswith power density on the-target,radio-frequencybias on the substrate,and substrate temperature being controlled interdependently for agiven'accumulation rate has not been previously accomplished.

OBJECTS OF THE INVENTION vlt is an object of this invention to producegarnet compositions by sputtering; 7 It is another object of thisinvention to produce gar- 'net compositions in an amorphous form byradiofrequency sputtering on Gadolinium Gallium Garnet -('GGG)substrates in a variety-of orientations.

' It is yet another object to convert the amorphous garnet compositionof the preceding object to epitaxial single crystal films-by heating.

It is anobject of this invention to produce amorphous garnet films atsubstrate temperatures less than 675C.

'lt is yet another object to prepare garnet compositions in amorphousform and in crystalline form from targets with compositions with excessof the resputtered species by adjusting the substrate temperature andradio-frequency bias on the substrate to produce stoichiometric garnetcomposition.

It is another object of" this invention to practice the precedingobjectby a' method which includes starting with a target compound of amixture of materials,

transporting these materials to an appropriate substrate where they arereacted to produce a garnet structure rapidly.

"It is an object ofthis invention to provide a method for growing anepitaxial layer of garnet by sputtering of the components thereof from atarget to a substrate c'o'ntrollably bysetting interdependently theoperasired layer according to the stoichiometry thereof varying'interdependently-the recited operational parameters'of thepreceding'object.

It is another object of this invention to practice the preceding objectby establishingthe' levels of 't he recited operational parameters for agiven accumulation rate of thelayer at the substrate.

It is another object of this invention to produce garnet films withsuitable properties for bubble domain applications.

It is another object of-thisinvention to provide the crystalline garnetfilm of the preceding object epitaxiallyin single crystalline form.

It is another object of this invention to' produce garnet film formagnetic bubble device by sputtering from a single target'where the filmis produced initially in amorphous form-and is then converted tocrystalline form. v 4 It isanother object of this invention to provide amethod for p'roducing a layer with a given composition by"radio-frequency sputtering said layer onto a substrate wherein-there isincluded varying interdepenature; radib frequency power to said target,and radiofrequency bias on said substrate, to establish said givencomposition of said layer on said substrate.

SUMMARY OF THE INVENTION It has been discovered for the practice of thisinvention that yield, stoichiometry and accumulation rate of a sputteredgarnet film can be controlled by interdependently varying magnitudes ofradio-frequency power at the target, and both temperature andradiofrequency bias on the substrate. In general, it has been determinedfor practice of this invention that garnet structure so produced issuitable for bubble domain device.

In the practice of one aspect of this invention, a mixed oxide target isutilized. In the practice of another aspect of this invention, astoichiometric compound garnet target may be utilized, but theaccumulation rate is less than for comparable growth conditions for anunreacted target. Practice of another aspect of this invention utilizesan enriched mixed oxide target wherein the component normally deficientin the produced film is an enriched species. The enriched target isreadily fabricated and the yield of the enriched species is predictable.The radio-frequency bias on the substrate removes the easily resputteredspecies from the growing film in a predictable way thereby producing theresultant stoichiometric garnet film.

Through the practice of this invention with mixed oxide componenttargets, desirable accumulation rate and stoichiometry of garnet filmare obtained. Therefore, use of an unreacted mixture of the oxides as atarget in radio-frequency sputtering obtains a substantial improvementin the rate of accumulation of the film compared to use of a reactedtarget of stoichiometric composition with time to grow garnet filmthicknesses less by approximately onethird.

By a practice of this invention there is a method of producingstoichiometric garnet structure film where A is a rare earth element ofthe lanthanide series, e.g.,,Gd, Eu, Tb, Er; B is Y, La or Sc; C is Alor Ga; and D is Fe, Ni or C0, through the steps of: establishing asingle source target; establishing radio-frequency power at target inthe range of approximately 5 watts/- sq.in. to I watts/sq.in.; andestablishing substrate temperature in the range approximately 0C to1,000C. In greater detail, it is beneficial to utilize a target forpractice of this invention which is a mixture of the components of saidfilm and radio-frequency power at the target in the range of 2watts/sq.in. to 23 watts/- sq.in. with a substrate temperature in therange of approximately 500C to 650C.

Specifically, for a grown garnet film of composition Y Ga Fe O where O y3, the target is preferably a composite mixture of the oxides Y O Ga Oand Fe O and is grown with radio-frequency bias on the substrate of lessthan approximately 200 volts d.c. level peak-to-peak, e.g., in the rangeof watts/sq. in. to I00 watts/sq. in. However, for some operationalcircumstance, the target may be of garnet structure.

In an illustrative example of the practice of this invention forobtaining a stoichiometric composition of A,B ,C,,D ,,O where O x 3 andO y 3, in a garnet film, the following are exemplary operationalparameters: temperature of the substrate is approximately in the range500C to I,OOOC; radio-frequency power to the target is approximately inthe range 35 watts/sq. in to 60 watts/sq.in; and the radio-frequencybias on the substrate is desirably in the range of 200 volts d.c. levelto 25 volts d.c. level, e.g., volts d.c. level, where the lower voltagemay suitably be below 25 volts. I

It has been demonstrated that D may be Fe in excess up to 27% of thetarget by weight composition. For an example of an excess of Fe in saidtarget of approximately 27% Fe, the power density in the target isapproximately 51 watts/sq. in. and the radio-frequency bias on thesubstrate is approximately in the range of volts d.c. level to 25 voltsd.c. level. For another example, the film may be deposited incrystalline form when the temperature on the substrate is greater thanapproximately 650C. For another example, the film may be deposited inamorphous form when the excess Fe is up to approximately 40% Fe and thetemperature is less than approximately 650C. It has also beendemonstrated that a variable substrate bias may be utilized whilesubstrate temperature is kept fixed.

Generally, in accordance with the objects of this invention, there isprovided a method for producing a layer with a given compositioncomprising the steps of: establishing a target which includes thecomponents of said layer and a substrate; radio-frequency sputteringsaid layer onto said substrate; establishing operational parameters suchthat said substrate is at a given temperature, radio-frequency power tosaid target is at a given power density thereat, and radio-frequencybias on said substrate is at a given d.c. level; and varyinginterdependently said operational parameters to establish said givencomposition of said layer on said substrate. The target may be anunreaeted mixture of different compounds of at least two components ofsaid layer, e.g., oxides of said components. Additionally, by varyinginterdependently said operational parameters, there may be obtained agiven accumulation rate of thickness of said layer on said substrate.

More particularly, in accordance with the objects of this invention,there is provided a method for producing a layer with garnetstoichiometry comprising the steps of: establishing a target whichincludes the components of said layer and a substrate with garnetcomposition; radio-frequency sputtering said film onto said substrateincluding, establishing operational parameters such that saidjsubstrateis at a given temperature, radio-frequency power to said target is at agiven power density thereat, and radio-frequency bias on said substrateis at a givendc. level; and varying interdependently said operationalparameters to establish a given accumulation rate of thickness of saidlayer on said substrate. Illustratively, when said temperature isestablished approximately in the range of 650C to ,l,000C, said powerdensity on said target is approximately in the range of 35 watts/sq. in.to 60 watts/sq. in., and said radio-frequency bias on said substrate isapproximately in the range of 200 volts d.c. level to 25 volts d.c.level; and said accumulation rate of thickness of said layer isapproximately in the range of 1 Angstrom/sec. to 3 Angstroms/sec.

DRAWINGS FOR THE INVENTION FIG. 1 is a schematic drawing partially insection and partially in circuit diagram of apparatus suitable forradio-frequency sputtering of material compositions according to theprinciples of this invention.

FIG. 2 is a schematic drawing illustrating the use of the apparatus ofFIG. 1 wherein alternatively a sub strate is in thermal contact with theanode electrode and wherein a substrate is thermally floating relativeto the anode electrode. FIG. 3 is a chart showing data in graphical formof the stoichiometric or molar ratio (Fe Ga,/Y plotted versus powerdensity on the target for several growth conditions in the practice ofthis invention.

FIG. 4 is a chart showing data in graphical form of accumulation rate onthe substrate versus power density on the target for radio-frequencysputtering according to the principles of this invention for targets of(a) powders of mixed oxides, Y 05 Ga Fe; 0 and (b) stoichiometriccompound Y Fe Ga 0, illustrating that the target of mixed oxides yieldsa faster accumulation rate on the substrate according to the principlesof this invention.

FIG. 5 is a graph showing data on accumulation rate.

versus radio-frequency bias on the substrate for an unreacted garnettarget with excess Fe and a particular power density on the targetillustrating that the accu-. mulation rate at the substrate decreaseswith increase of the radio-frequency bias.

FIG. 6 presents data in graphical form of molar ratio of the grownstoichiometric garnet film according to the principles of this inventionillustrating that the molar ratio is higher for a gallium-backedsubstrate at z 450C than for a substrate thermally floating at z 900C.

FIG. 7 is a line diagram showing an idealized plot of terrnperatureversus time illustrating the amorphous to crystalline transformation ofa garnet film according to the principles of this invention.

GENERAL DESCRIPTION OF APPARATUS FOR THE PRACTICE OF THE INVENTION Aschematic diagram of apparatus suitable for the radio-frequencysputtering of garnet materials according to the principles of thisinvention is shown in FIG. 1.

Generally, the cathode holder 3 and the anode 25 are arranged in a diodeconfiguration. A shutter 47 is placed between the cathode and anode andaxial and horizontal motion are provided thereto to cover the substrateduring presputtering of the target and also to protect the target duringsputter cleaning of the substrate prior to deposition. Heating of thesubstrate is obtained by resistance heaters 43 embedded in the anodeassembly. The structure 41 is generally constructed of stainless steel.The molybdenum substrate holder is precoated with garnet'ofapproximately the target composition to prevent resputtering ofmolybdenum on the substrate 15. The radio-frequency power is splitbetween the target 26 and the substrate 15 by the network 5 Y whichprovides independent radiofrequency bias control on the substrate.

The target is made of oxides pre-reacted to form the garnet compoundwhich are ground, mixed and fired several times and then formed andfired into a disc. The preferable type of target is unreacted andcomprised of the metal oxides which are mixed and pressed at highpressure and temperature and are only slightly'sintered.

Generally, both reacted and unreacted targets have stoichiometriccomposition Y Fe;, Ga, O, Additionally, a suitable target may also beunreacted GdzGazYlG' with excess Fe of up to approximately 40%compensation, e.g., 27% Fe. lllustratively, the substrate showed singlecrystal electron diffraction pat terns with Kikuchi lines, indicating asurface nearly free from lattice distortion. Surfaces with the l I l], 1l0], l00]orientations were. found suitable for growth of epitaxialsingle crystal films of garnet stoichiometric composition.

SPECIAL DESCRIPTION OF APPARATUS FOR PRACTICE OF THE INVENTION FIG. 1depicts an exemplary radio-frequency sputtering system, suitable forproducing garnet film in accordance with the principles of thisinvention. As shown in the figure, an RF source 1 is coupled to targetholder assembly 3, via the impedance matching network 5 and couplingcapacitor 7. Impedance matching network 5 comprises variable capacitors5a and 5b and inductor 5c and is employed to match the impedance of thesputtering system 2 to the impedance of RF source 1. During RFsputtering, it is possible to control to some extent the properties ofthe deposited films by applying a radio-frequency bias voltage to thesubstrate 15. For that purpose, there is provided a network comprisingAC impedance elements 9, 11, and 13 which ensure that an appropriatebias is on substrate 15.

Inductor 9 is coupled at one end point to the node be tween impedancematching network 5 and coupling capacitor 7, and at another point tomovable wiper arm 17 with contact 18. The center of inductor 9 isgrounded and lower end 19 is floating, with the wiper arm 17 contact 18on inductor 9 acting to provide a complete path. The wiper arm 17 iscoupled to variable capacitor 11 and variable inductor 13 which iscoupled to capacitor 21. Meter 23 is coupled to the junction betweencapacitor 21 and anodic substrate holder 25 via inductor 27 and isemployed to measure the peak-topeak radio-frequency voltage bias on theanodic substrate holder. Inductor 27 and capacitor 29 are employed toisolate the meter 23 from the radio-frequency bias on the substrate.

The grounded center tap inductor 9 inverts the RF voltage so that whenthe RF signal from source 1 is positive, the voltage between thegrounded center tap and the floating end of inductor 9 is negative andvice versa. When wiper arm 17 is moved on inductor 9, the amplitude ofthe inverted voltage is varied. Variable capacitor l1 and variableinductor 13 are employed to tune the phase shift network, which may bevaried over 360C. Thus, both the amplitude of the RF substrate bias andthe phase thereof may be adjusted. Therefore, the RF load, i.e., theimpedance of the sputtering system, varies in accordance with thesputtering system parameters. Thus, where it is desirable to obtain amaximum peak-to-peak voltage on the substrate 15, for a particular wiperarm 17 setting, the entire radiofrequency circuit is adjusted toresonance.

Substrate 15 is mounted on substrate holder 25. Target 26 is mounted inconductive relationship with target holder assembly 3. Cathode targetholder assembly 3 is water cooled, with water entering and exiting viapipes 4 in accordance with the arrows shown at the top of the assembly.Respective Helmholtz coils 31 which are energized by a direct voltagesource, not shown, surround the cathode target holder assembly 3 andpedestal portion 51 of anode substrate holder 25. Coils 31 provide amagnetic field of intensity approximately 30 to 80 gauss perpendicularto the plane oftarget 26 and substrate 15. This magnetic field increasesthe concentration of electrons in the sputtering environment to increasesputtering efficiency. Additionally. the magnetic field acts to increasethe bias on the substrate 15. Ground shield 33 around target 26 limitsand focuses the sputtering to the control portion of target 26. Shield33 is removably affixed to mount 35 to permit changing of targets 26.

Ceramic sleeves 37 and 39 insulate the. cathode target holder assembly 3from the metal sputtering chamber 41 and the housing portion 35a ofmount 35, respectively. Heating assembly 43 maintains'the substrate 15at the desired temperature. Cooling coils 45 cool the anode structureand therefor the substrate 15. Shutter arrangement 47 is movablypositionable between substrate 15 and target 26. Turning knob assembly49 which is external to chamber 41 permits movement of the shutter 47from the region between substrate 15 and target 26. Pedestal portion 51of the anode substrate holder is mounted upon insulation 53 to isolateelectrically the substrate holder assembly from metal chamber 41.

The sputtering chamber incorporates a titanium sublimation pump 55surrounded by liquid nitrogen container 57. Before the sputtering isinitiated, the sublimation pump getters active species, such as carbonand carbon-bearing compounds, from within the chamber onto the surfaceof the cryogenically cooled drum-59 of the pump. Titanium filament 61 isenergized viaan electrical source, not shown. High purity oxygen enters-at port 63 and is passed through the"titanium pu'mp'in to thesputtering chamber. The high purity oxygen becomes further purified inthe titanium pump and is used to sputter-clean the surface of substrate15.

"Sputtering chamber 41 is pre-pu mped down-'. to a pressure'of from 2.0to 8.O l" Torr while substrate is maintained at the desired temperaturefor sputtering. This pre-pumping is achieved via the port at lower rightof chamber 41. lllustratively, a freon-cooled diffusion pump may be usedto achieve'the desired vacuum in the sy'steml The system is thenbackfilled-with high purity oxygen until a pressure of approximately2X10 Torr is reached. The oxygen is scrubbed in the titanium sublimationpump and the sputtering system is further readied for sputtering.

'Next, theshutter 47 is positioned between the target 26 and substrate15 by knob 49 and the circuitry-power is turned on. Oxygen isestablished in chamber 4l,via

port 63. Oxygen plasma is generated and components of the target aresputtered upon shutter 47. Usually a pre-sputtering period of from 5 tominutes is-adequate, and it has been found that the average time foreffective pre-sputtering is about 15 minutes.

Then, the substrate 15 is sputter cleaned in an oxygen environment. Thesystem is first pumped down againto the base pressure of 2Xl0' Torr andis back-filled with oxygen. Simultaneously, radio frequency voltage 18is applied to produce a dclevel bias on the substrateof approximately150 volts. With shutter=47 in conductive contact with shield 33, whichis grounded via the walls of mount and chamber 41, there is sputtercleaning Torr may be employed.

PRACTICE OF THE INVENTION FIG. 2 is a schematic diagram illustratingfunctionally a portion of the apparatus to indicate two ways that asubstrate 15 may be mounted on anode 25 for the practice of thisinvention. Substrate 15a is shown supported via a layer of gallium,i.e., it is gallium-backed, by modybdenum block 25; and substrate 15b isshown as thermally floating, i.e., it is placed on block 25 and issomewhat insulated therefrom by the lack of effective thermal controlthereto. Consequently, substrate is at the same temperature z 450C asthe block 25, whereas substrate 15b reaches a higher temperature z 900C,i.e., it is thermally floating. Thus, the Gd Ga O substrate wafers 15were either gallium-backed to the molybdenum substrate holder or allowedto float thermally as illustrated in FIG. 2. The gallium backedsubstrate 15a is within a few degrees of the temperature of themolybdenum holder. The temperature of a substrate 15b as measured byoptical pyrometry reached 900C and greater depending on the power inputto the target; Identical atom flux from the target impinged on thehigher temperature floating substrate 15b and on the lower temperaturegallium backed substrate 15a.

For an exemplary film growth procedure, with reference to FIGS. 1 and 2,the system is pumped to below 10 Torr, and pure oxygen is admitted intothe system to about 2.5X10 Torr, which is maintained by pumping. Withtarget 26 and substrate 15 shielded and the anode 25 grounded, thesystem is pre-sputtered for about 30 minutes. Thengthe shield 47 ismoved from target region to allow sputter cleaning of the substrates forabout 10 minutes. The shutter shield 47 is then opened therebypermitting atom flux from the target to reach the substrates 15a and15b. The radio-frequency bias on the substrate 15 is adjusted to apredetermined level and deposition occurs over a period, e.g., up toapproximately 38 hours.

After deposition, the grown films are annealed in flowing oxygen in anopen tube furnace. Desirable results were obtained when the grown filmswere maintained from 900C to l,l00C for 24 hours, and then slowlycooled, e.g., 50C/hr. Circular erruptions occurred in thefilm whenrapidly heated to annealing temperature, whereasfilm slowly heated toannealing temperature showed tensile cracking. However, by selecting anintermediate rate of heating to annealing temperature, large nearlyperfect films were obtained as regards such circular erruptions andtensile cracks. Proper lattice matching between the substrate andgarnetfilm. reduces the tendency for cracking to take place.Illustrative thicknesses of grown films, as measured. by. aTaylor-Hobson Tally vSurf instrument, ranged from 0.01 to 15 microns, ofwhich I to 5 micron films were typical. Optical absorption methods werealsoemployed in determining film thickness and the results comparedclosely. to the mechanical measure I EXEMPLARY DATA FOR Tin; INVENTIONExemplary garnet films according tothe principles of this inventionwereexamined from various interdependent operational parameters of thesputtering 'of radiofrequency .bias on the substrate; power density on.the target, substrate temperature and accumulationrate of the film atthe substrate. The film composition as determined by electron microprobeanalysis; was normalized to 3 moles of Yttrium. The ratio of the sum ofthe normally occupied Fe sites to therare earth sites in a material withgarnet composition 1 was considered as the stoichiometric or molarratio.

For comparison, the molar ratio for the ideal stoichiometric compositionis 1.667. FIG. 3 is a plotof the molar ratio as a function; of powerdensity in watts/iii on the target showing results of reacted andunreacted targets either at --450C or z 900C.

Both types of targets for the data of FIG. 3 havethe stoichiometriccomposition Y Fe Ga ,O The substrates were gadolinium gallium garnet GdGa O Substrates that are gallium-backed and at z 450C are compared'tothermally floating examples which are at temperatures from 850C to 980C.For comparison purpose, films were deposited on both lowertemperaturesubstrate 15a and higher temperature substrate 15b of FIG. 2simultaneously. The samples at z 450C are amorphous and reddish whilefilms deposited at z 900C arecrystalline and range in color from amberto greenish-yellow.

The difference in composition between films depos- 10 within 2% of thestoichiometric composition of the target, whereas for the reacted garnettarget there was an accumulation rate of about 0.4A/sec with greaterthan I I 1% deviation from the target composition.

With reference to FIGS. 5 and 6, unreacted targets "with: 27% excess Fe-(Gd Y ,Fe Ga 0, were also 'mally'fl'o'ating 'at temperature 900C. FIG.5 shows the accumulation rate on the substrate as a function of Thefilms on the Ga-backed z 450C substrates'are amorphous and nonmagneticand upon crystallization ited on the high temperature z 900C substratefilms deposited on and the lower temperatures= 450C substrate isconsiderably smaller when'produced from the unreacted target than whenproduced from the reacted target. The unreacted target produces anapproximately constant differential' in composition between the highertemperature substrate and the lower temperature substrate whereasv thereacted compounded garnet target resulted in films that vary widely incomposition. As the power density on the target is decreased, thecomposition of the grown filrriapproaches more closely that of thetarget. Theoretically, the tendency for the convergence of compositionsof the higher and lower temperature substrates at lower'powers resultsbecause the heating of the'thermally isolated. substrates is decreasedto a'point where the films are deposited in amorphous form atapproximately "the temperature of the substrate-holder. Sputteringatrelatively low power density on the target produced more nearly idealfilm stoichiometry. Films prepared from the unreacted, i.e., mixedcomponent, target approach the target stoichi ometry before the reacted,i.e.,.com-pounded garnet, target. Therefore, 'there is an improvement-inthe ratio of the species sputtered fromtheunreacted-targetcompared tothe species sputter from the reacted target.

FIG. 4 shows the accumulation rate on the substrate as a function ofpower density for boththe unreacted target and the reacted target.Illustratively, the accumulation'rate from the unreacted targetexceeds'the accumulation rate for the reacted target by a factor ofabout 2 to 3. Illustratively, at watts/in on the target with substratesat 450C, sputtered films were prepared from an unreacted target with anaccumulation rate on the substrate of 1.5A/sec. Such films were becomesingle crystalline garnet structures; occasionally small amounts of asecond phase were found and phase appearing occasionally.

' The data of FIGS. 5 and 6 show the trends of' composition ofthefilrnproduce'd and accumulation rate on the substrate withradio-frequency bias on the substrate and indicate a large influence ofthe bias on film composition, particularly at relatively hightemperature. Therefore, it has been determined for practice of thisinvention that radio frequency bias control of the substrate isadvantageous for the fabrication of stoichiometric films ofmulti-component systems.

FIG. 6 presents plots of molar ratio versus radio- .frequency bias onthe substrate for both galliumbacked substrates 450C and thermallyfloating substrates 900C. Generally, it is shown by FIG. 6 thatunreacted targets withexce ss Fe can be used to prepare stoichiometricgarnet compositions and structures by adjusting the power density to thetarget which can be generally greater than to a stoichiometric targetand also by adjusting the temperature range at which stoichiometricfilms can be prepared as well as the bias that -is applied to thesubstrate. It will therefore be understood from the foregoing discussionthat the interrelationship among target composition, power density,substrate temperature and substrate bias can be selectively ,controlledto obtain stoichiometric garnet compositions and structures.

CONSIDERATIONS FOR THE INVENTION tering. It has been demonstrated thatcomposition control may be successfully achieved for single crystal gar-.net film at substrate temperatureshigher than 500C,

which is the upper'temperature limit given in the noted prior art US.Pat. No. 3,607,698 for the preparation of single crystal GdzGazYIGfilms.

. In thepracticeof this-invention, a garnet structure is -;deposited asan amorphous layer on a GGG surface and then transformed to crystallineform by heating as illustrated by FIG. 7. The growth takes placeoutwardly from the interface between the film and the substrate towardthe surface of the film with resultant epitaxial single crystal garnetlayer on the substrate. The epitaxial growth process via the amorphousto crystalline transformation is a relatively low temperature processfor epitaxial growth of garnet films, when compared to the prior artpractice. The typical thermal history of garnet films undergoingamorphous to crystalline transformation is shown in FIG. 7. For Y Fe GaO film on Gd Ga O substrate, the amorphous to crystalline transformationtemperature is between 650C and 675C for a film prepared at z 450C.Amorphous garnet films deposited on sapphire substrates showed anamorphous to crystalline transformation temperature higher than for GGGsubstrate, and the transformation temperature varied from 50C to 100Cgreater than the transformation temperature of film on GGG.Theoretically, these differences in crystallization temperatures are dueto differences in the interfacial surface energy between the substrateand the film. The amorphous garnet films are a deep burgundy red intransmitted light and after crystallization range from a yellowgreen toan amber-yellow depending on composition and thickness of the film.

Amorphous garnet films deposited in accordance with the principles ofthis invention have been found to be readily etched in dilute l-lCl,whereas the crystallized garnet deposited in accordance with theprinciples of this invention is only slightly soluble in dilute HCl.Patterns may readily be etched in the amorphous garnet, thenrecrystallized to form epitaxial garnet patterns.

Exemplary technology for providing special amorphous to crystallinetransformation to obtain single crystal film is disclosed in copendingapplication Ser. No. 319,125 filed Dec. 29, 1972 by P. Chaudari, et al.,and commonly assigned.

We claim:

1. Method of producing film with garnet stoichiometric composition A B CD a o where O x 3 and O y 3, wherein A is a rare earth element of thelanthanide series including Gd, Eu, Tb and Er;

B is Y, La or Sc;

C is Al or Ga; and

D is Fe, Ni or C; comprising the steps of:

a. establishing a target with substantially non-garnet structure andwith said stoichiometric composition and establishing a substrate withgarnet composition;

b. sputtering said film onto said substrate from said target includingestablishing operational parameters by:

1. establishing said substrate at a temperature approximately in therange of greater than 500C to 1,000C,

2. establishing radio-frequency power to said target approximately inthe range of 2 watts/sq. in. to 100 watt/sq. in;

3. establishing radio-frequency bias on said substrate at a given d.c.level; and

c. varying interdependently said operational parameters to deposit ontosaid substrate a film having substantially said garnet stoichiometiccomposition and to establish a given accumulation rate of thick- LIIness of said film on said substrate.

2. Method as set forth in claim 1 wherein 1. said temperature of saidsubstrate is established approximately in the range of greater than 500Cto 650 C, and

2. said radio-frequency power to said target is established in the rangeof 2 watts/sq. in. to 23 watts/sq.

3. Method as set forth in claim 2 including the step of establishingsaid radio-frequency bias on said substrate less than approximately 200volts d.c. level peakto-peak.

4. Method as set forth in claim 1 and wherein said radio-frequency powerto said substrate is established approximately in the range of 35watts/sq. in. to 60 watts/sq. in. i

5. Method as set forth in claim 4 wherein said radiofrequency bias onsaid substrate is varied during production of said film thereon whilesaid temperature of said substrate is maintained at an approximatelyfixed value.

6. Method as set forth in claim 1 wherein power density on said targetis approximately in the range 20 watts/sq. in. to 100 watts/sq. in.

7. Method as set forth in claim 1 wherein said temperature of saidsubstrate is greater than approximately 650C,

said radio-frequency bias on said substrate is approximately volts d.c.level.

8. Method as set forth in claim 1 wherein said temperature of saidsubstrate is less than approximately 650C.

9. Method for producing a layer with garnet stoichiometry comprising thesteps of establishing a target which has substantially a garnetstoichiometric composition which is substantially nongarnet structureand establishing a substrate with garnet composition,

radio-frequency sputtering said film onto said substrate from saidtarget including,

establishing operational parameters such that said substrate is at agiven temperature approximately in the range of 650C to 1,000C,radio-frequency power to said target is at a given power density thereatapproximately in the range of 35 watts/sq. in. to 60 watts/sq. in., andradio-frequency bias on said substrate is at a given d.c. levelapproximately in the range of 200 volts d.c. level to 25 volts d.c.level, and varying interdependently said operational parameters todeposit onto said substrate a layer having substantially said garnetstoichiometric composition and to establish a given accumulation rate ofthickness of said layer on said substrate approximately in the range of1 Angstrom/sec. to 3 Angstroms/sec.

10. Method as set forth in claim 9 wherein said target is comprised of amixture of oxides Y 0 Ga O and Fe O and said film is produced withstoichiometric composition of 11. Method of claim 9 wherein said targetis an unreacted mixture of different oxides of at least two componentsof said layer.

1. said temperature of said substrate is established approximately inthe range of greater than 500*C to 650 *C, and
 1. Method of producingfilm with garnet stoichiometric composition AxB3 xCyD5 yO12, where O < x< 3 and O < y < 3, wherein A is a rare earth element of the lanthanideseries including Gd, Eu, Tb and Er; B is Y, La or Sc; C is Al or Ga; andD is Fe, Ni or Co; comprising the steps of: a. establishing a targetwith substantially non-garnet structure and with said stoichiometriccomposition and establishing a substrate with garnet composition; b.sputtering said film onto said substrate from said target includingestablishing operational parameters by:
 1. establishing said substrateat a temperature approximately in the range of greater than 500*C to1,000*C,
 2. establishing radio-frequency power to said targetapproximately in the range of 2 watts/sq. in. to 100 watt/sq. in; 2.Method as set forth in claim 1 wherein
 2. said radio-frequency power tosaid target is established in the range of 2 watts/sq. in. to 23watts/sq. in.
 3. Method as set forth in claim 2 including the step ofestablishing said radio-frequency bias on said substrate less thanapproximately 200 volts d.c. level peak-to-peak.
 3. establishingradio-frequency bias on said substrate at a given d.c. level; and c.varying interdependently said operational parameters to deposit ontosaid substrate a film having substantially said garnet stoichiometiccomposition and to establish a given accumulation rate of thickness ofsaid film on said substrate.
 4. Method as set forth in claim 1 andwherein said radio-frequency power to said substrate is establishedapproximately in the range of 35 watts/sq. in. to 60 watts/sq. in. 5.Method as set forth in claim 4 wherein said radio-frequency bias on saidsubstrate is varied during production of said film thereon while saidtemperature of said substrate is maintained at an approximately fixedvalue.
 6. Method as set forth in claim 1 wherein power density on saidtarget is approximately in the range 20 watts/sq. in. to 100 watts/sq.in.
 7. Method as set forth in claim 1 wherein said temperature of saidsubstrate is greater than approximately 650*C, said radio-frequency biason said substrate is approximately 75 volts d.c. level.
 8. Method as setforth in claim 1 wherein said temperature of said substrate is less thanapproximately 650*C.
 9. Method for producing a layer with garnetstoichiometry comprising the steps of establishing a target which hassubstantially a garnet stoichiometric composition which is substantiallynongarnet structure and establishing a substrate with garnetcomposition, radio-frequency sputtering said film onto said substratefrom said target including, establishing operational parameters suchthat said substrate is at a given temperature approximately in the rangeof 650*C to 1,000*C, radio-frequency power to said target is at a givenpower density thereat approximately in the range of 35 watts/sq. in. to60 wAtts/sq. in., and radio-frequency bias on said substrate is at agiven d.c. level approximately in the range of 200 volts d.c. level to25 volts d.c. level, and varying interdependently said operationalparameters to deposit onto said substrate a layer having substantiallysaid garnet stoichiometric composition and to establish a givenaccumulation rate of thickness of said layer on said substrateapproximately in the range of 1 Angstrom/sec. to 3 Angstroms/sec. 9.METHOD FOR PRODUCING A LAYER WITH GARNET STOICHIOMETRY COMPRISING THESTEPS OF ESTABLISHING A TARGET WHICH HAS SUBSTANTIALLY A GARNETSTOICHIOMETRIC COMPOSITION WHICH IS SUBSTANTIALLY NONGARNET STRUCTUREAND ESTABLISHING A SUBSTRATE WITH GARNET COMPOSITION, RADIO-FREQUENCYSPUTTERING SAID FILM ONTO SAID SUBSTRATE FROM SAID TARGET INCLUDING,ESTABLISHING OPERATIONAL PARAMETERS SUCH THAT SAID SUBSTRATE IS AT AGIVEN TEMPERATURE APPROXIMATELY IN THE RANGE OF 650*C TO 1,000*C,RADIO-FREQUENCY POWER TO SAID TARGET IS AT A GIVEN POWER DENSITY THEREATAPPROXIMATELY IN THE RANGE OF 35 WATTS/SQ. IN. TO 60 WATTS/SQ. IN., ANDRADIOFREQUENCY BIAS ON SAID SUBSTRATE IS AT A GIVEN D.C. LEVEL TO 2APPROXIMATELY IN THE RANGE OF 200 VOLTS D.C. LEVEL TO 25 VOLTS D.C.LEVEL VARYING INTERDEPENDENTLY SAID OPERATIONAL PARAMETERS TO DEPOSITONTO SAID SUBSTRATE A LAYER HAVING SUBSTANTIALLY SAID GARNETSTOICHIOMETRIC COMPOSITION AND TO ESTABLISH A GIVEN ACCUMULATION RATE OFTHICK-
 10. Method as set forth in claim 9 wherein said target iscomprised of a mixture of oxides Y2O3, Ga2O3 and Fe2O3, and said film isproduced with stoichiometric composition of Y3Fe5 yGayO12, where 0 < y <3.